Photoinduced graft copolymerization of vinyl monomers onto deoxy(thiosulfato)chitin (S2O3-chitin) has been studied. Chitin was first tosylated and subsequently transformed into S2O3-chitin. S2O3-chitin has good solubility over tosylchitin. Graft copolymerization of S2O3-chitin proceeded very easily by ultraviolet irradiation without catalyst. Photolysis of S2O3 groups was confirmed by infrared spectra. But the photolysis occurred only in quartz, not in a Pyrex tube. Methyl methacrylate (MMA) and acrylonitrile showed good grafting activities. In the case of acrylic acid and acryl amide, homopolymer formation was predominant, and the degree of grafting was low. We chose the MMA monomer for further information. The grafting rate of S2O3-chitin using MMA was much faster than those of chitin and O-acetylchitin. Under the appropriate conditions, the degree of grafting reached 600% only in 2 h, and the grafting efficiency was over 75% in any monomer concentration. But addition of DMSO into the polymerization system decreased the degree of grafting.